The manufacture of urea prills in a prill tower is a well known process. Typically, molten urea is sprayed through nozzles located at the top of the tower. The molten droplets fall down the tower while being contacted counter-currently with a gas stream, usually air. The airstream provides a cooling effect that solidifies the urea droplets into spherical form. The urea prill is then collected at the bottom of the tower.
Urea prills are manufactured in various sizes, such as feed-grade prills which have particle sizes below U.S. 20 mesh in diameter and the larger fertilizer and forest-grade prills. Depending upon the size of the prills and the temperature and humidity of the gas stream, the prill may not completely solidify as it falls through the tower. U.S. Pat. No. 3,533,776 and U.S. Pat. No. 3,334,160 describe prilling towers having a fluidized bed located at the bottom of the tower to further provide cooling and completely solidfy the urea prill.
The use of upflowing air countercurrent to the prill is thought to provide optimum cooling. This method of prilling, however, presents serious dust collection problems. The air, carrying with it the dust generated from prilling, exits the top of the prilling tower. It is both expensive and difficult to design and maintain large dust collection systems at the top of such towers. It has been necessary for some prilling operations to actually decrease the design rate in order to reduce dust emissions from such prilling operations.
One effort to reduce the dust problems associated with prilling can be found in U.S. Pat. No. 3,059,280. This patent discloses a method of prilling urea in a co-current stream of air that allows treatment of the dust-laden air at ground level. A major problem with this invention, however, is that co-current air reduces the residence time of the falling urea particle and thus the particle may not be completely solidified by the time it reaches the bottom of the tower. With larger particles, only the outside surface may be solid, having a molten core. This type of prill will disintegrate and generate even more dust when hitting the bottom of the tower for collection.
The present invention eliminates the cost and mechanical disadvantages of treating the dust-laden air at the top of the tower and provides a means for using co-current air for making all sizes of urea prills by utilizing both co-current and countercurrent airflow in a single prill tower.